TUNICATA. 



1221 



tions follow each other somewhat rapidly, and 

 have all the same direction ; but suddenly they 

 are arrested, and then recommence in a con- 

 trary direction. The blood thus sent sometimes 

 from behind forwards, and sometimes vice 

 versa, ascends towards the thorax ; but does 

 not appear to be conducted thither by vessels. 

 It is poured out between the inner tunic of the 

 abdomen and the viscera lodged in that cavity ; 

 and here it forms currents, which vary in their 

 position according as the movements of the 

 animal, or any other mechanical causes, op- 

 pose their passage. In general, however, the 

 chief portion of the blood ascends by the 

 dorsal or the ventral surface of the abdomen ; 

 and after having bathed the surface of the 

 viscera, it gains the base of the branchial sac. 

 When the heart's contraction is from behind 

 forward, the ascending current passes along 

 the anterior side of the abdomen, and the 

 blood enters a large vertical canal, on the 

 front of the respiratory cavity, termed by 

 Milne-Edwards the great thoracic or ventral 

 sinus. This median sinus gives rise on each 

 side to a series of large transverse vessels, 

 which intercommunicate by means of a num- 

 ber of minute vertical vessels, and which, 

 after having formed a kind of vascular net- 

 work, spread over the walls of the branchial 

 cavity, and terminate in another vertical canal 

 parallel to the ventral sinus, but situated on 

 the opposite side of the thorax. A portion 

 of the blood arrives at the same time in this 

 dorsal sinus without having traversed the 

 branchial net-work, by means of a vessel that 

 arises from the superior extremity of the great 

 ventral sinus and surrounds the base of the 

 branchial orifice. Lastly the blood spreads 

 out between the viscera and the internal tunic 

 of the body, descends along the dorsal side of 

 the abdomen, and again reaches the heart. 

 If the circulation were constant in the above 

 direction, it would somewhat resemble that of 

 the other Acephalans. The heart might then 

 be compared to an aortic ventricle, the tho- 

 racic sinus to a great branchial artery, and the 

 dorsal sinus to a branchial vein. But owing 

 to the contrary directions of the blood-cur- 

 rents, from the periodically varying impulses 

 of the heart, the vessels that fulfil at one 

 time the functions of veins, at another become 

 arteries, and vice versa. 



This peculiar extra-vascular circulation, so 

 well described by Milne-Edwards, is also very 

 distinctly seen in the Clavellince. The learned 

 professor especially notices C. nana as highly 

 illustrative of this phenomenon, the inter- 

 abdominal space being in this case very large, 

 and the currents of the nutritive fluid, with 

 its suspended spherical globules, being easily 

 discernible through the transparent integu- 

 ments. 



Mr. Bowerbank has favoured me with an 

 account of some observations made by him on 

 the circulation in the common tegumentary 

 mass of Botryllus. Under the microscope, 

 the more transparent portion of the test ex- 

 hibited a reticulated arrangement of sangui- 

 f'erous channels or vessels ; perhaps the true 



" marginal vessels " of Savigny ; each mesh, 

 formed by the anastomosing currents, bt-in^ 

 occupied by one of the star-like " systems " of 

 animals. No communication could be traced 

 between the circulation and that of the ani- 

 mals themselves ; the former appearing to be 

 analogous to the peculiar stem-circulation of 

 the polypifera, to which also the circulation of 

 the nutrient fluid in the budless stems of 

 Perophora, noticed by Mr. Lister, vide supra, 

 has reference. 



Embryogenesis of the Botryllidts. In the de- 

 velopment of the ova of the compound Asci- 

 dians, there are certain striking differences from 

 the conditions that take place during the em- 

 bryo-genesis oftheAscidiadce. These, however, 

 are chiefly confined to the composition of the 

 egg, the formation of the Blastoderm, the mode 

 of growth of the caudal appendage, the organs 

 of vision, and the anterior appendages. From 

 the elaborate observations of this family, 

 given by Professor Milne-Edwards in his 

 Paper before referred to*, it appears that 

 the ova of several species of the PolycUnina 

 are, whilst still enclosed in the ovary, and 

 before that their development is much ad- 

 vanced, of an ellipsoid form, and are com- 

 posed of a very thin external membrane, a 

 subgelatinous whitish and granular inner mass, 

 and a minute central vesicle filled with a 

 watery fluid. The internal vesicle is the 

 vesicle of Purkin je, or the proligerous vesicle ; 

 the granular substance surrounding the ve- 

 sicle is the imperfect vitellus, the vitelline 

 membrane being the external envelope. 



Whilst these ova are still enclosed in the 

 upper part of the post-abdomen, they grow 

 rapidly and become spherical. But the most 

 remarkable change that takes place consists 

 in the colour of the vitellus, which is at first 

 a pale, and afterwards a deep yellow. The 

 vesicle of Purkinje is still visible at the com- 

 mencement of this period of the development, 

 but it soon disappears, and there then appears 

 on the surface of the vitellus a nebulose spot 

 of pale yellow, which appears to be the blasto- 

 derm or proligerous layer destined to become 

 the embryo of the young Ascidian. 



The ova arrive in the cloaca, and sometimes 

 are even lodged in the lateral portions of the 

 thoracic chamber, without having undergone 

 any other appreciable modification. M. Milne- 

 Edwards considers it probable that the fecun- 

 dation of the ova takes place in the interior 

 of this cavity. They are here brought into 

 contact with the spermatozoa, and very shortly 

 after having arrived here, they exhibit evi- 

 dences of active internal changes. 



The granules composing the vitelline mass 

 become grouped into clusters, forming them- 

 selves, as it were, into balls, and giving the 

 surface an embossed or mulberry- like aspect. 

 At the same time there is formed between the 

 yolk and the external membrane of the ovum, 

 a gelatinous, transparent, and nearly colourless 

 layer, which apparently becomes the external 

 tunic or test of the young animal. 



* Observations sur les Ascidies composees. 

 4i 3 



